U.S. Pat. No. 3,970,943 which issued July 20, 1976 to Everett E. Chapman, et al., for "Analog-to-Pulse Integrating Converter" is a typical patent of apparatus for converting an analog signal to pulses whose rate is proportional to the analog signal. The analog signal is integrated, and after the integrated signal reaches a predetermined amplitude, a pulse is generated. Two level detectors are used, one for positive values of the integrated signal and one for negative values of the integrated signal to signal a flip-flop to initiate the output pulse train. When an output pulse is produced, a pulse being a predetermined charge content is delivered back to the input of the integrator, such pulse being of an amplitude and polarity to oppose the effect of the incoming signal. The amount of charge in the feed-back pulse is controlled by a precision power supply and two precision switches, one for positive and the other for negative pulses. When the switches are switched, there is a substantial voltage across the switch, and the use of two precision power supplies and two precision switches require that the power supplies and switches be matched.
There are a number of circuits using the same concept but with various embellishments and improvements.
One of the problems with such apparatus is in establishing precisely the amount of charge in the pulse delivered back to the integrator, and an associated problem is in causing the amount of charge in negative and positive fed-back pulses to be precisely the same.
Still another problem when both positive and negative pulses are fed-back is that the switching from a source of pulses of one polarity to that of another occurs when a relatively high voltage is across the switch, thereby requiring a high voltage switch. When two precision power supplies and two switches are used, both the power supplies and the switches need to be matched.